Several advances in pacing have occurred over the years by providing improved methods in sensing natural pacing rhythms. For instance, in demand pacing devices, the objective is to provide stimulatory pulses in the absence of the natural heartbeat. That is, the pacemaker or pacemaker/cardioverter/defibrillator is designed to deliver a pulse at a fixed rate as long as no natural heartbeat is sensed. Sensing of the natural frequency of heartbeats can be done to accommodate changes in the natural pacing frequency such as during natural rhythms of sleep or exercise.
Benefits have recently been identified that tend to promote introducing a circadian variation to the rate-adaptive pacemaker base rate, i.e., lowering the base heart rate during sleep or during prolonged periods of inactivity. Several pacemakers or pacemaker/cardioverter/defibrillator are currently available that have two basal rates to more closely match diurnal or circadian heart rate variations (by programming two resting rates). U.S. Pat. No. 3,921,642 to Preston et al. discusses the advantages of providing a pacemaker capable of searching for and detecting the occurrence of natural resting basal heart rates within a predetermined range. U.S. Pat. No. 3,593,718 and in European Patent Application No. 0 089 014 describe pacemakers that respond to changes in respiration rate, for instance during exercise. Alternate means for sensing physical activity and adjusting the pacemaker rate accordingly are described in U.S. Pat. No. 4,776,338.
Clinical evidence is available that tends to show that patients with decreased heart rate variability die earlier than those with normal variability and are a predictor of arrhythmic cardiac death, myocardial infarction, rapid progression of atheroscerosis and death from heart failure. A possible correlation has been identified between sedentary lifestyle and risk of ventricular arrhythmias based on a comparison of occurrences of ventricular arrhythmias in healthy active vs. sedentary men, and men with previous myocardial infarction. Accordingly, the greatest number and highest grades of ventricular arrhythmias during exercise were found in healthy sedentary men.
It is also well know that naturally the heart goes through varied basal rates. For instance, during normal sleep patterns, the heart rate changes depending on the sleep state (e.g., REM sleep, etc.). That is, normally the heart rate is not fixed at a particular rate during sleep.
There is a growing population of patients having implantable pacemaker or pacemaker/cardioverter/defibrillator devices who are largely sedentary and who are therefore likely to be paced at their basal rate for much of the day, since they are unable to achieve any measurable amount of exercise on their own. However, since current adaptive rate pacemakers or pacemaker/cardioverter/defibrillators are designed to find a range of natural rhythms occurring in the patient, whether to slow them during sleep, or to increase the rate during physical activity, with the goal of sensing and establishing a pacing rate within a controlled range of preexisting rates, currently available pacemakers or pacemaker/cardioverter/defibrillators do not address the benefit of having periods of elevated pacing designed into the pacemaker, particularly where no natural rhythm for the elevated pacing rate has been established.